In an increasingly violent society, businesses and government institutions are subject to a greater number of threats against both life and property. Such threats may be in the form of ballistic threats, explosive blasts, forced entries, as well as others. Security measures have been taken to protect against such threats. These include the installation of special windows that have increased strength, to withstand an attack. For example, windows that have security glazings that can resist certain explosive blasts, ballistic threats, and/or forced entry threats are being specified in new commercial, as well as industrial buildings.
An explosion is an extremely rapid release of energy in the form of light, heat, sound, ground shock wave and a progressive air blast shock wave. The shock wave consists of highly compressed air traveling radially outward from the source at supersonic velocities. As the shock wave expands, pressures decrease (with the cube of the distance), and when it meets a surface in line-of-sight of the explosion, it is reflected and can be amplified by several times. These pressures decay rapidly with time (i.e., exponentially) and last a very brief time, measured typically in thousandths of a second, or milliseconds. Diffraction effects, due to the presence of reentrant corners or edges of the building, may act to confine the air-blast, increasing its duration. Late in the explosive event, the shock wave becomes negative, creating suction. Behind the shock wave, where a vacuum has been created, air rushes in to fill the vacuum, creating high intensity wind or drag pressure on all surfaces of the building. It is this drag pressure that is responsible for propelling flying debris in the vicinity of the detonation. For an external explosion, a portion of the energy is also imparted to the ground, creating a crater and generating a ground shock wave analogous to a high-intensity, short duration earthquake.
The shock wave is the primary damage mechanism of an explosion. The pressure it exerts on building surfaces may be several orders of magnitude greater than the loads for which the building is designed. The shock wave also acts in directions, which the building may not have been designed for, such as upward on the floor system. In terms of sequence of response, the air-blast first impinges on the weakest point in the vicinity of the device closest to the explosion, typically the exterior envelope of the building, and usually the window and/or door locations are the first to fail prior to progressive wall collapse. The explosion initially pushes on the exterior walls at the lower stories and may cause window breakage and/or wall failure. As the shock wave continues to expand, it enters the structure, pushing both upward and downward on the floors.
Glass is often the weakest part of a building, breaking at low pressures compared with other components such as the floors, walls, or columns. Past incidents have shown that glass breakage may extend miles for large external explosions. This is due to the seismic loading or shock wave that propagates by particle velocity. High velocity glass fragments have been shown to be a major contributor to injuries in such incidents. For incidents within downtown city areas, falling glass poses a major hazard to passersby and prolongs post-incident rescue and clean up efforts by leaving tons of glass debris on the street.
For an explosive threat defined by its charge weight in pounds of TNT equivalent, W, and its distance from the target, or stand off, R, the peak pressure and impulse of the shock wave are evaluated using scaling charts available in military handbooks. The impulse is defined as the area under the pressure verses the time curve (i.e., the integral of pressure with respect to time). The impulse is an indicator of how long the air-blast acts on the target, information that is needed for evaluating its response. The duration of the loading, td, may be defined as the duration of a linearly decaying function having the peak impulse, I, and pressure, P, of the actual air-blast (i.e., td=2I/P). Because this duration differs somewhat from the actual duration (which is based on an exponentially decaying function), it is referred to as an “equivalent” duration. Windows that are designed to withstand such explosive blasts may also present better resistance to natural disasters such as hurricanes, tornadoes, and severe storms.
Conventional windows that call for security glazings have a primary frame to secure a glazing unit, within a defined opening of a building, for example. The frame is referred to as a “primary” frame because it may be the only frame that is needed to close the given opening between a “threat side” and a “safe side”. Where the threat side is outside of the building, and the safe side is inside the building, the primary frame serves not only to secure the glazing, but to also weatherproof the opening.